1. Field of the Invention
This invention relates to a method and a device for magnetizing a magnet system and, for example, is suitable for magnetizing and magnetically anchoring permanent magnets of rare-earth materials on the rotor of an electric motor which may be applied in automatic magnetization installations with low cycle times or with large-scale manufacture.
2. Discussion of Related Art
It is known to use a magnetization coil for magnetizing permanent magnets. The magnetization coil is arranged directly above or around the magnet body to be magnetized. A charged capacitor is allocated to the magnetization coil and the capacitor is discharged via the coil. The magnetic field, which is built up for a brief period in the magnetization coil, magnetizes the magnet body. In order to build up a sufficiently large magnetic field one must use a magnetization coil with many windings or with a large inductance. The usual pulse durations are 10 ms or more. With this, the magnetization coil is heated to an undesirable extent, which renders a high cycle frequency impossible and necessitates the application of expensive cooling systems.
An electrical pulse generator suitable for the operation of magnetization devices according to the known type is disclosed in the German Patent Reference DE-28 060 00. This pulse generator contains a circuit for energy recovery with two capacitors or two simultaneously triggered high-current switches.
Permanent magnets of rare-earth metals such as neodymium-iron-boron (NdFeB) are now taking the place of the ferrite magnets which are applied in large numbers and are considerably more difficult to magnetize because of their high coercive force. Although a magnetic field strength of 800 kA/m is sufficient for the magnetization of conventional magnets of magnet alloys or ferrites, the modern magnets demand 1600-4000 kA/m and have a field strength that lies higher than the saturation degree of all known ferromagnetic materials. An inclusion of iron for the magnetization coil therefore at the most only has an assisting effect, but may no longer effect a field concentration. Air-core coils must be used for magnetization and have a considerably worse efficiency on magnetization because the magnetic field may not be concentrated on the magnets. Thus, considerably higher outputs need to be brought into the coil, and their undesired heating is accordingly higher.
Conventional magnetization installations operate with pulse durations of 10 ms or more. Such pulse durations result in sufficient penetration depths of the magnetic field also in electrically conductive materials where the propagation of magnetic fields is delayed because of eddy currents and also permit the application of inexpensive electrolyte capacitors for storing energy for the magnetization pulse and the application of semiconductor switches for the mains frequency. This technology is suitable for individual magnetizations in the laboratory and in the field of manufacture, but not for large-scale manufacture. In large-scale manufacture there is not sufficient available time for cooling the magnetization coil between the individual magnetization procedures. For modern permanent magnets with a high coercive force the power of such a magnetization installation is limited in large-scale manufacture.
With a restricted space for the magnetization coil, the magnets in the assembled condition may hardly be magnetized with conventional methods. In this case, previously magnetized permanent magnets are installed into the magnet system, which places particular demands on the assembly. The handling of magnetized permanent magnets and magnet systems is awkward because ferromagnetic particles of all types are attracted and may hardly be removed again. The same is the case with the peeling or spalling of the magnet which inevitably results when there is impact of the permanent magnets.
The arrangement for magnetizing magnet systems disclosed in the German Patent Reference DE-100 49 766 makes do without magnetization pulses. According to this reference, a magnetization coil constructed of a coolable high-temperature superconductor is used, which is fed by a direct-current source capable of being closed-loop controlled. This arrangement requires an expensive cooling and consumes much energy. The magnetization coil of a high-temperature superconductor is expensive and is prone to malfunctioning.
German Patent Reference DE-39 34 691 describes a device with which the magnets are inserted into a conductor through which current flows. A magnetization of pre-assembled magnets may not be achieved with this device. The parallelization mentioned in German Patent Reference DE-39 34 691 relates to conductors lying next to one another, for magnetizing long rod magnets or for multi-pole magnetization.